Frequently in semiconductor manufacturing, the starting quality of incoming silicon material may have a profound effect on the final electrical device quality. A silicon wafer characteristic of interest is surface micro-roughness, sometimes called "haze", measured by a topographic inspection device, such as a surface scanner. Surface roughness is of interest in other industrial manufacturing processes, such as disk and optical component fabrication, particularly with increasing demands for quality control. Since topographic inspection instruments function at various spatial bandwidths, with varying response functions, it has been difficult to characterize these instruments since each may be reporting a very different value from the same surface. The range of spatial wavelengths observed by the instrument generally means the data sampling rate per unit distance. Sometimes the spatial bandwidth is expressed as spatial frequency and computed from the one dimensional grating equation: ##EQU1## where f is the spatial frequency, .theta..sub.i is the angle of incidence of a monochromatic light beam, .theta..sub.s is the principal scattering angle from the grating, and .lambda. is the wavelength of the beam. Alternatively, the ASTM has suggested a definition of spatial frequency in terms of a frequency range of a component of the Fourier transformed surface profile of an object. Calibration targets have been built to help obtain standard readings from some devices, particularly scanners. Such targets have been built to replicate haze patterns present on new, unpatterned, polished wafer surfaces.
The surface of a calibration target or standard should be extremely uniform and isotropic over a zone of interest, should be readily reproduced, and should have extremely small surface features to read the extremely low level values of haze which replicate the surface of prime silicon wafers. Such a haze standard was described by Scheer in U.S. Pat. No. 5,198,869, "Reference Wafer for Haze Calibration". The device described there however, envisions a haze standard that reads at a much higher level than would be useful for relating to a prime silicon wafer. The standard described was very useful but was comprised of two materials, one of which is a film layer causing additional effects from the optical path length differences through the film as well as phase shifts at the interfaces. Both of these effects will change with illumination wavelength and should therefore be eliminated. The reading device is a light collector of the type described in U.S. Pat. No. 4,597,665, assigned to Tencor Instruments, although other beam reading devices could be used. Various types of features have been used to simulate haze or roughness including pits, step height bars, line-space pairs and grid patterns. In most instances, these features were fabricated by photolithography on silicon substrates.
Calibration targets for another application of interest simulate a magnetic disk surface texture. Such targets must deal with a predominant mechanical effect that can reduce the reliability, or functionality, of disk recording media, referred to as stick-slip, or, in its worst manifestation, blocking. Stick-slip is due to a high coefficient of friction that causes irregularities in the rotational speed of rigid recording disks. Since the system is dependent on a steady rate of data from the recording medium, stick-slip invariably will result in loss of data integrity. Blocking, on the other hand, can cause the heads to adhere completely to the disk surface. A final phenomenon, related to blocking, is called stiction and may be described as occasional blocking. This again will lead to premature mechanical wear.
All three of these effects may occur if the surfaces of the recording head or the media surface are too finely polished. To avoid these problems, a defined amount of surface texture must be imparted onto the surfaces themselves. This is known in the prior art. The amount of this imparted surface texture must be carefully controlled because it is desired to have the head in the required close proximity to the recording surface. This texture may be modelled with a calibration target and observed by a surface topographic inspection instrument.
Consequently, there is a need for a calibration target which models the tribological properties of the head-disk interface, particularly the surface topography of the disk coating.
An object of the invention was to devise a calibration target which would replicate the micro-roughness of highly polished wafer or disk surfaces at the lower limits of step height resolution, i.e. atomic distances.